There are procedures in many fields including manufacturing, assembly, repair, and surgery in which it is useful to align a tool with a target path, feature, or other target location that may be hidden from view or not sufficiently visible to the user. For example in airframe assembly and repair it may be necessary to drill a hole through a layer of material in line with an existing hole in an inaccessible component positioned behind the material. For another example many surgical procedures require a surgeon to align a tool, such as a drill, a guide wire driver, a bone saw, or an ultrasound probe with a target path that cannot be sufficiently marked or seen. The target path may be an ideal plane or trajectory through tissue as determined in preoperative planning or determined using intraoperative techniques, for example an estimated centerline of an anatomical feature such as a femoral neck as described by Hodgson in international patent publication WO/2006/133573. The target path may also be related to features of an implant, such as a fixation screw hole in a bone plate or IM (“IM”) nail, wherein the objective is to align a drill, guide wire driver, or other tool relative to the hole when the hole or the direction of the hole is hidden from view.
Some examples of surgical procedures which can be facilitated by guidance to establish a desired tool alignment are:                femoral head resurfacing. This procedure involves inserting a guide wire through the femoral neck along a target path at a planned angle and location.        pedicle screw fixation in spine surgery where a drill, drill-guide, guide wire or screw must be inserted along a target path, within a known structure (a pedicle) and for a limited depth to avoid injury to surrounding structures outside the bone.        placement of ilio-sacral screws in pelvic bone surgery, where a drill, drill-guide, guide wire or screw must be inserted along a target path (thru the iliac bone, within the sacral ala and vertebral body) and for a limited depth to avoid injury to surrounding structures outside the bone.        osteotomy (cutting of a bone), where orientation of a surgical saw to be used to cut a bone in a specific planned orientation would be critical to the next steps of the procedure: realignment of bones, placement of implants (e.g. knee arthroplasty)        placement of hip or knee arthroplasty implants in a planned orientation (e.g placement of an acetabular component in hip arthroplasty)        locating fixed solid organs (e.g. liver, pancreas, kidney, or other) or mobile hollow organs (e.g. bowel, bladder) for percutaneous placement of a stent or device or for establishing access to a planned location which is hidden from view. (e.g. nephrostomy tube, percutaneous endoscopic gastrostomy tube, hepatic, biliary or pancreatic diverting stent) or for sampling a mass or tissue at a location hidden from view but known relative to a probe, for the purpose of biopsy.        
Furthermore in many procedures such as drilling or cutting with a tool, the user may find it useful to know how far a tool has progressed along a target path from a starting point, for example to know how deep a hole has been drilled or in order to select the correct length of screw, pin, or the like to install. For example in various surgical procedures, surgeons may want an estimate of the correct length of a screw to be installed in a drilled hole such that the screw spans the bone at the hole location but does not protrude excessively from the bone into the surrounding tissue.
Also in many tool alignment procedures, the user may need to use the tool in various orientations relative to the user's point of view. The user may also prefer to hold the tool in their right or left hand, which may affect the orientation of the tool and the visibility of the tool and the alignment target area. For example in many surgical procedures the surgeon may need to use a tool in various orientations to gain access to working space and a clear tool path, for example to have the tool pass by the non-involved limb of the patient, the operating table, and the various limb holders, bolsters and the like that are used in surgery.
Generally, tool guidance and navigation procedures require some form of user interface and feedback, such as a visual display, to provide targeting information to the user.
Intramedullary nailing (“IM nailing”) is one example of a surgical application in which it is necessary to align a tool with hidden features. In the following detailed description IM nailing is provided as a non-limiting example to illustrate application of various aspects of the invention.
To stabilize a fractured long bone, surgeons usually insert an IM nail (“IM nail”) along the medullary canal of the bone. To hold the distal section of the fractured bone, distal locking screws are installed transverse to the axis of the bone and passing through holes in the distal end of the IM nail. Installing the distal locking screws creates a challenge for the surgeon because the locking screw holes are inside the bone and cannot be seen. An IM nail may also distort unpredictably as it is pushed distally down through the bone and as the bone fragments are aligned, therefore the position of the distal locking holes may be difficult to determine using guides attached to the proximal end of the nail.
Surgeons commonly locate the distal locking holes by trial and error using hand-held guide wires or a drill and a series of x-ray images taken during the operation. The main tool for acquiring these images is a C-arm fluoroscope, which is typically moved incrementally until the holes appear as circles in the image, thus indicating that the fluoroscope is aligned with the distal locking holes. Then the drill bit or drill guide is typically positioned on the skin surface over the area of the hole and adjusted, using more images, until it is centered and aligned with the hole. This method is time-consuming and exposes the surgical team and patient to radiation.
Although the radiation dose a surgeon receives from a C-arm fluoroscope has generally been considered safe, there is some disagreement about this. Hafez (2005) estimates that radiation doses recorded at the fingertips are as much as seventy five times higher than doses recorded at the base of the fingers. Cumulative exposure to radiation may be a concern particularly for trauma surgery teams.
Computer assisted techniques, making use of electromagnetic position tracking technology to assist with IM nailing surgery, are described in Krause, U.S. Pat. Nos. 6,074,394 and 6,503,249; Govari, U.S. Pat. No. 7,060,075; and Ritchey, US published application 20100274121. A navigation system (Trigen Sureshot™ Distal Targeting System, Smith & Nephew, Memphis Tenn. USA) is commercially available. These systems use electromagnetic navigation systems (comprising a field generator that emits a controlled magnetic field, at least one sensor that responds to the magnetic field by generating a signal indicative of the sensor's position relative to the field generator, a computer, and associated software), a drill guide, and a targeting display to show the user the relative locations of the drill guide and the sensor such that the user can align the drill guide to a predetermined position relative to the sensor. Some systems described in the prior art include an electromagnetic sensor located in the implant at a known location relative to the features to be targeted (in the case of IM nailing, the distal locking holes) throughout the targeting procedure. Ritchey, WO2010/129141 describes various methods and apparatus for estimating the travel of a drill bit through a drill guide.
Most modern, widely used, IM nails are cannulated along their length, the cannulation having a circular cross-section and a diameter related to the overall size of the nail. Typical IM nails have various holes and slots, in addition to the locking holes, located along the length of the nail. Typically the nail is implanted by attaching an insertion tool to the proximal end of the nail and passing the nail cannulation over a guide wire. The guide wire is then withdrawn and the nail may be hammered in further, rotated, withdrawn, or otherwise positioned as required using a variety of fittings attached to the insertion tool. In some systems an electromagnetic sensor tool is inserted into the cannulation at a position that is known relative to the locking screw holes.
The systems described by Krause and Govari, and the Sureshot™ system, include a separate drill guide which would typically be held by the surgeon with one hand, while he or she holds a drill in their other hand.
In such systems using a separate drill guide, the drill bit slides through the guide in a direction along the drill bit axis. In prior art systems having a field generator separate from the drill and the drill guide, the drill, drill guide, and drill bit all may move in and out of, and move about within, the measurement range of the field generator. When the field generator is integrated with or attached to the drill guide in a fixed position, as shown in some prior art systems, the drill bit slides in and out of the measurement range of the field generator during drilling.
In many surgical procedures, including IM nailing, it is desirable to position tools with sub-millimetic and sub-degree accuracy (Beadon 2007). Electromagnetic navigation systems can be affected by the presence of certain metals (particularly ferromagnetic and electrically conductive materials) and magnetic fields located in and nearby the measurement range of the field generator (Kirsch 2005; Beadon 2007). Many drills, including commonly used surgical drills, contain ferromagnetic and conductive parts, and may also contain electric motors which may contain magnets and which may generate magnetic fields during operation. Drill bits commonly used in surgery are made of ferromagnetic materials such as hardened stainless steel, which, when moved within the range of the electromagnetic tracking equipment, may cause distortion of the electromagnetic fields and may cause inaccurate tracking measurements. There may also be variations in the particular field generator and environment that affect the accuracy of tracking.
In typical electromagnetic position tracking systems, the sensor coordinate system in which the system reports the position and orientation of a sensor is defined by the relative location and the characteristics of components inside the sensor tool. These are variable in manufacture. For example in a cylindrical sensor tool, the sensor coordinate system as manufactured may have an axis only approximately coaxial with the cylindrical axis. To achieve an accurate known relationship of the coordinate system to the physical shape of the sensor tool, a set of correction factors may be determined by calibrating each individual sensor tool in a calibration fixture at manufacture, and writing the correction factor to a memory device built in to the sensor (Aurora™ Tool Design Guide Rev. 3 Dec. 2005 Northern Digital Inc. Waterloo, Ontario, Canada). This individual calibration and programming process, along with a suitable memory device, generally increases the manufacturing cost of the sensor tool.
When attaching a sensor tool to an implant in order to target features in the implant, the accuracy to which the relative position of the sensor coordinate system and the features is known directly affects the accuracy of targeting. This relative position may be included in a database stored in memory, and recalled if the user correctly indicates the type of sensor and implant being used, provided the database of implant dimensions includes that particular implant. In this case, the manufacturing tolerances of the implant, the sensor tool, and any other component used to position the sensor tool all become direct factors in targeting accuracy. For example with an IM nail, if the sensor tool attaches to the insertion tool which is in turn attached to the proximal end of the nail, as shown in certain embodiments described by Ritchey in patent application WO2010/129141, the manufacturing tolerances of the handle, the distance from the proximal end of the nail to the locking holes, and the sensor tool length may all contribute to targeting variance.
With electromagnetic position tracking systems, measurement errors may occur if external magnetic fields are present or objects made of certain metals are brought into the range of the field generator (Kirsch 2005). Such distortions can be unpredictable and may not be apparent to the user during navigation. For example measurements may appear steady, but be biased several millimeters in a particular direction by the presence of a ferromagnetic tool, such as a surgical hammer, located close to the field generator.
Outputs of typical electromagnetic position tracking systems can include low frequency, high amplitude measurement noise. Such noise can cause measurement values to vary. It is also typical for these systems to occasionally fail to return a valid reading for a sensor which can cause the user display to freeze momentarily until good data is received again. Small, lightweight field generators and small sensors are especially prone to produce orientation data having occasional outlying values.
Finally, in certain cases and with certain types of IM nailing procedures, the preferred practice is to drill through the proximal holes and lock the proximal bone fragment to the nail prior to drilling and locking the distal holes (e.g. see TFN™ Titanium Trochanteric Fixation Nail System; Technique Guide. Synthes GmbH, Oberdorf, Switzerland). In these cases the proximal locking screws block the nail cannulation and make it impossible to install a sensor tool that passes through the cannulation past the proximal locking screws, for example for the purpose of targeting distal locking holes.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.